Spray dry vs freeze dry
Spray dry is around 4-5 times more economical than that freeze dry as the former consumes less electricity with drying time of 5-100s, whereas freeze dry can 1-3 days. Spray-dried product is highly stable, because of its low moisture content and water activity. Typically, moisture is about 2-5%. At such conditions, the products are rather resistant to microbiological and oxidative degradation.
Co-current & counter current flow in spray dry
The co-current direction (hot air and direction of spray) is preferable for heat sensitive bio-active compounds, in which feed is atomised prior to contact with drying air (150oC-220oC) flow. The final powder is exposed to moderate temperature of 50oC-80oC, which limits the thermal degradation. Counter-current is preferable if achieving better energy efficiency is more important than preservation of heat sensitive bio-active compounds. In general, higher spray drying temperature increases productivity (throughput) while lowering solid particles’ bulk density as faster water evaporation impedes proper shrinkage of the spheres and induces pore formation within.
Glass transition temperature in spray dry
Sugar-rich materials can be difficult to spray dry directly without a carrier agent due to their stickiness behaviour and low glass transition temperature (from a hard and relatively brittle state into a viscous/rubbery state at increasing temperature). Low glass transition temperature leads to wall deposition problems, caking, lumping, chemical changes, and drying difficulties. Three factors influence the glass transition temperature of a product: temperature, humidity and product composition.
The glass transition happens, when the temperature rise over a range of 10oC to 20oC above the glass transition temperature of the product. Moisture content also plays an important role in reducing the glass transition temperature. Even though most of the powder products have less than 5% moisture content, a small increase of moisture content (e.g. 1%) can dramatically decrease the glass transition temperature.
Carriers should have the following properties: (i) highly soluble in process solve, (ii) good film forming ability, (iii) low viscosity even at high concentration, (iv) high glass transition temperature, and (v) high molecular weight.
Carrier’s effects on products include: (a) increases solubility, (b) reduces hygroscopicity, (c) increase product recovery, (d) protects of bioactive compounds, (e) reduces moisture content, (f) reduces water activity, (g) increases glass transition temperature, and (h) decrease particle size.
Common carriers are gum arabic, maltodextrins, gelatin, starches, pectin, methyl cellulose, alginates, tricalcium phosphate and their combinations. Combination is used to combine the best properties of two (or more) different types of carriers, for example gum Arabic and starch. Starch has low film forming ability but high molecular weight and high glass transition temperature; while gum has strong film forming ability (better drying properties and preservation of bioactive compounds) but comparatively lower glass transition temperature.